Nitrocarburized microalloyed steel member

ABSTRACT

A nitrocarburized microalloyed steel member consists of a microalloyed steel that includes a nitrocarburized layer on a surface, a cross-sectional structure of which steel except for the nitrocarburized layer includes a ferrite and pearlite structure. The microalloyed steel mainly consists of Fe and has a composition: C having a content of 0.30 mass % or more and 0.50 mass % or less; Si having a content of 0.05 mass % or more and 0.30 mass % or less; Mn having a content of 0.50 mass % or more and 1.00 mass % or less; S having a content of 0.03 mass % or more and 0.20 mass % or less; Cu having a content of 0.05 mass % or more and 0.60 mass % or less; Ni having a content of 0.02 mass % or more and 1.00 mass % or less; and Cr having a content of 0.05 mass % or more and 0.30 mass % or less. If the contents of the Cu, the Ni, and the Cr are represented by WCu, WNi, and WCr mass %, respectively, and composition parameters F 1  and F 2  are 185WCr+50WCu and 8+4WNi+1.5WCu−44WCr, respectively, then the composition parameters F 1  and F 2  satisfy F 1&gt; 20 and F 2&gt; 0.

RELATED APPLICATIONS

This application claims the priorities of Japanese Patent Application No. 2005-379203 filed on Dec. 28, 2005 and Japanese Patent Application No. 2006-138299 filed on May 17, 2006, which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a microalloyed steel member having a nitrocarburized layer formed on a surface.

2. Description of the Related Art

Patent Literature 1: Japanese Patent Application Laid-Open No. 09-030632

Patent Literature 2: Japanese Patent Application Laid-Open No. 06-128690

Patent Literature 3: Japanese Patent. Application Laid-Open No. 05-279795

Patent Literature 4: Japanese Patent Application Laid-Open No. 05-279794

Patent Literature 5: Japanese Patent Application Laid-Open No. 09-324258

Patent Literature 6: Japanese Patent Application Laid-Open No. 2002-226939

Patent Literature 7: Japanese Patent Application Laid-Open No. 2005-264270

An automobile crankshaft is used in an environment in which a high torsional load and a high bending load repeatedly act on the crankshaft. The crankshaft is, therefore, required to be excellent in static strength and fatigue strength. On the other hand, since the crankshaft is a member quite large in size and complicated in shape, it is normally and basically manufactured using a microalloyed steel that is not quenched and tempered after being hot-forged. In this case, to secure strength, it is necessary to finally perform a hardening treatment on a surface of the steel. The Patent Literatures 1 to 4 disclose methods using a nitrocarburizing treatment as the surface hardening treatment. The nitrocarburizing treatment is a treatment in which a workpiece is treated in, for example, an ammonia gas atmosphere at a temperature equal to or lower than the A1 transformation point or generally at a temperature of about 570 degrees Centigrade, part of carbon as well as nitrogen is introduced into the steel, nitrides or carbides are produced, and a surface layer of the steel is thereby hardened. Such a nitrocarburizing treatment is suited for, for example, mass-production of crankshafts that are large-sized engine parts of an automobile since the treatment hardly generates strains in the workpiece differently from a carburizing and quenching method, and does not require a long time differently from a nitriding method.

Meanwhile, for the crankshaft manufactured using the nitrocarburizing treatment, it is essential to perform a straightening treatment thereon after the nitrocarburizing treatment so as to straighten a bend generated during forging or nitrocarburizing. Conventionally, with a view of ensuring an excellent straightenability, a normalizing treatment for size regulation for grains of the steel structures and elimination of strains is executed after the hot forging. However, the number of steps increases by as much as the added normalizing treatment, resulting in cost increase. Patent Literatures 5 to 7, therefore, disclose microalloyed steels each of which can ensure high straightenability even if this normalizing treatment is omitted.

Each of the conventional microalloyed steels having the improved straightenability has, however, a disadvantage of an insufficient bending fatigue strength. In addition, if a hardness of the nitrocarburized layer is increased to increase strength, the microalloyed steel falls into a dilemma that the initially intended straightenability is deteriorated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a nitrocarburized microalloyed steel that can suppress an increase in hardness to such an extent that a straightening treatment after a nitrocarburizing treatment can be easily performed on the steel even if a normalizing treatment after hot-forging is omitted, and that can ensure sufficiently high bending fatigue strength.

To attain this object, according to a first aspect of the present invention, there is provided a nitrocarburized microalloyed steel member consisting of a microalloyed steel that includes a nitrocarburized layer on a surface, a cross-sectional structure of the steel except for the nitrocarburized layer including a ferrite and pearlite structure, wherein

the microalloyed steel mainly consists of Fe and has a composition:

C having a content equal to or more than 0.30 mass % and equal to or less than 0.50 mass %;

Si having a content equal to or more than 0.05 mass % and equal to or less than 0.30 mass %;

Mn having a content equal to or more than 0.50 mass % and equal to or less than 1.00 mass %;

S having a content equal to or more than 0.03 mass % and equal to or less than 0.20 mass %;

Cu having a content equal to or more than 0.05 mass % and equal to or less than 0.60 mass %;

Ni having a content equal to or more than 0.02 mass % and equal to or less than 1.00 mass %; and

Cr having a content equal to or more than 0.05 mass % and equal to or less than 0.30 mass %, and wherein

if the contents of the Cu, the Ni, and the Cr are represented by WCu, WNi, and WCr mass %, respectively, and composition parameters F1 and F2 are 185WCr+50WCu and 8+4WNi+1.5WCu−44WCr, respectively, then the composition parameters F1 and F2 satisfy: F1>20  (1) and F2>0  (2).

In the present invention, the “steel mainly consisting of Fe” means that a remainder of the steel except for the various added elements that can be contained by the steel as described in the present specification consists of Fe and inevitable impurities.

If this steel composition is adopted and the steel is air-cooled after being hot-forged, a structure of the steel naturally offers a ferrite and pearlite structure. Even if a normalizing treatment after hot-forging is omitted, the straightening treatment can be performed on the nitrocarburized steel. In addition, if the Cu, the Ni, and the Cr are contained in the steel so that the composition parameters F1 and F2 satisfy the expressions (1) and (2), it is possible to ensure sufficiently high bending fatigue strength while suppressing a hardness of the nitrocarburized layer formed on the surface from being excessively increased.

In the composition of the microalloyed steel adopted in the present invention, the Cr, the Cu, and the Ni are particularly important added elements. Among these elements, the Cr forms high-hardness Cr nitrides, and increases the hardness of the nitrocarburized layer. In this case, if an addition amount of the Cr is excessively increased, then the Cr nitrides are excessively formed, the hardness of the nitrocarburized layer is excessively increased, and a straightenability of the steel member is considerably deteriorated. However, the inventor of the present invention discovered that newly addition of the Cu and the Ni so that the composition parameters F1 and F2 satisfy the expressions (1) and (2) enables greatly increasing the fatigue strength while suppressing the increase in hardness (or realizing a satisfactory straightenability while maintaining the fatigue strength at a certain degree), and finally completed the present invention.

The expressions (1) and (2) are experimentally discovered by the inventor of the present invention. The expression (1), i.e., 185WCr+50WCu>20  (1)′ represents conditions for adding the Cr and Cu to secure the fatigue strength. In addition, the expression (2), i.e., 8+4WNi+1.5WCu−44WCr>0  (2)′ represents conditions for adding the Ni, the Cu, and the Cr to secure the straightenability. A coefficient of the Cr addition amount WCr is a quite high positive value in the expression (1)′ whereas it is a high negative value in the expression (2)′. This obviously indicates that the increase in hardness by formation of the Cr nitrides greatly contributes to securing the high fatigue strength ((1)′), but that the increase in hardness conversely contributes to deteriorating the straightenability. Therefore, if the WCu, WCr, and WNi are set so as to satisfy both the expressions (1)′ and (2)′, the steel member capable of ensuring both the high fatigue strength and the high straightenability, in other words retaining the satisfactory straightenability while maintaining the fatigue strength at a certain degree, can be realized.

What should be noticed herein is, above all, the effect of adding the Cu. A coefficient of the Cu addition amount WCu is, differently from the Cr addition amount WCr, positive both in the expressions (1)′ and (2)′. This signifies that the addition of the Cu can improve both the fatigue strength and the straightenability. This is considered to be due to the fact that the Cu contributes to improving the fatigue strength by a mechanism different from that of the Cr. Namely, during the straightening treatment, it is necessary to leave a necessary deformation on not only a bulk steel material but also the nitrocarburized layer on the surface thereof for the straightening even after the machining load is eliminated, that is, to generate a plastic deformation thereon. FIG. 2 is a typical representation of a stress-strain curve of the material. It is known that a hardness of the material reflects a maximum stress expressed on the stress-strain curve, that is, a tensile strength σT. To increase the hardness, i.e., to increase the tensile strength σT corresponds to the need of a higher machining stress so as to generate the same plastic deformation or strain, and this means that the straightenability is deteriorated (as indicated by a broken line A in FIG. 2). However, if a yield stress σY can be increased while suppressing the increase in hardness (as indicated by a one-dot chain line B in FIG. 2), then the fatigue strength can be increased without greatly deteriorating the straightenability, (in other words the same yield stress can be realized at a lower degree of the hardness, that means improving the straightenability while maintaining the yield stress (or fatigue strength) at a certain degree). In the opinion of the inventor of the present invention, the Cu added to the steel inhibits the plastic deformation (that is, profile shift and proliferation) of a steel matrix of the nitrocarburized layer in some way or the other (e.g., solid-solution hardens the matrix), and increases the yield stress.

On the other hand, the Ni does hardly contribute to improving the fatigue strength as is obvious from the fact that the Ni is not present as a parameter in the expression (1)′. However, it is obvious from a coefficient of the Ni added amount WNi in the expression (2)′, the Ni has the effect of improving the straightenability twice as high as that of the Cu. The reason is considered as follows. The Ni is an austenite stabilizing element (note that an austenite is easier to plastically deform than a ferrite), and the Ni added to the steel improves ductility of the steel matrix of the nitrocarburized layer and, therefore, suppresses propagation of a fatigue crack.

The steel composition and reasons for limiting numerical parameters adopted in the present invention will now be described.

“C having a content equal to or more than 0.30 mass % and equal to or less than 0.50 mass %.”

The C is a necessary element to secure the strength. However, if the C content is less than 0.30 mass %, the strength cannot be secured. If the C content exceeds 0.50 mass %, the hardness of the material before cutting is excessively increased. This deteriorates cutting machinability. The C content is more preferably equal to or more than 0.31 mass % and equal to or less than 0.45 mass %.

“Si having a content equal to or more than 0.05 mass % and equal to or less than 0.30 mass %.”

The Si is contained in the steel as an element serving as a deoxidizer during the heat solution treatment and serving to improve the fatigue strength. If the Si content is less than 0.05 mass %, desired effects cannot be produced. If a large amount of the Si is added so that the Si content exceeds 0.30 mass %, a ferrite phase is hardened and straightenability is deteriorated. The Si content is more preferably equal to or more than 0.06 mass % and equal to or less than 0.28 mass %.

“Mn having a content equal to or more than 0.50 mass % and equal to or less than 1.0 mass %.”

The Mn is an element that increases the fatigue strength and that is essential to generate Mn-containing sulfides contributing to improving the machinability. If the Mn content is less than 0.50 mass %, a generation amount of the Mn-containing sulfides is insufficient and the insufficient machinability is provided. On the other hand, if the Mn content exceeds 1.0 mass %, a volume percentage of generation of the pearlite is excessively high, resulting in deterioration in straightenability. The Mn content is more preferably equal to or more than 0.55 mass % and equal to or less than 0.95 mass %.

“S having a content equal to or more than 0.03 mass % and equal to or less than 0.20 mass %.”

Similarly to the Mn, the S is an element that is essential to generate the Mn-containing sulfides contributing to improving the machinability. If the S content is less than 0.03 mass %, the generation amount of the sulfides is insufficient and the insufficient machinability is provided. On the other hand, if the S content exceeds 0.20 mass %, a toughness and a ductility of the steel are deteriorated. In addition, cracks and the like tend to occur during the hot-forging and the fatigue strength is reduced. The S content is more preferably equal to or more than 0.04 mass % and equal to or less than 0.15 mass %.

“Cu having a content equal to or more than 0.05 mass % and equal to or less than 0.60 mass %.”

As stated, the Cu exhibits the effects of improving the fatigue strength and improving the straightenability. If the Cu content is less than 0.05 mass %, the effects of the Cu are not conspicuous. If the Cu content exceeds 0.60 mass %, a hot workability of the steel is deteriorated. The Cu content is more preferably equal to or more than 0.10 mass % and equal to or less than 0.50 mass %.

“Ni having a content equal to or more than 0.02 mass % and equal to or less than 1.00 mass %.”

As stated, the Ni has the effect of improving the straightenability. If the Ni content is less than 0.02 mass %, the effect of the Ni is not conspicuous. If the Ni content exceeds 1.00 mass %, bainite is generated to thereby increase the hardness. As a result, the effect of improving the straightenability is rather hampered, resulting in the deterioration of the machinability. The Ni content is more preferably equal to or more than 0.05 mass % and equal to or less than 0.60 mass %.

“Cr having a content equal to or more than 0.05 mass % and equal to or less than 0.30 mass %.”

The Cr has effects of increasing an internal hardness and a surface layer hardness after nitriding, and of improving the fatigue strength. If the Cr content is less than 0.05 mass %, the effects of the Cr are not conspicuous. If the Cr content exceeds 0.30 mass %, then the surface layer hardness is considerably increased, resulting in the deterioration of the straightenability. The Cr content is more preferably equal to or more than 0.08 mass % and equal to or less than 0.25 mass %. “F1=185WCr+50WCu>20 F2=8+4WNi+1.5WCu−44WCr>0”

To secure the fatigue strength, the parameter F1 is set to fall within the above-stated range. To secure the straightenability, the parameter F2 is set to fall within the above-stated range. FIG. 3 is a stereoscopic representation of three component ranges of Cu, Cr, and Ni in a three-dimensional coordinate space (a projected plan from an Ni axis side), with a horizontal axis indicating the Cu content, a vertical axis indicating the Cr content, and an axis (Ni axis) at a right angle with respect to a drawing sheet indicating the Ni content. These component ranges are represented as an internal region of a solid figure surrounded by the following six planes in the three-dimensional coordinate space: 8+4WNi+1.5WCu−44WCr=0, 185WCr+50WCu−20 =0, WCu=0.05, WCu=0.6, WNi=0.02 and WNi=1.0. An upper limit of the parameter F1 obtained by linear programming is 84.21 (WCr=0.293 and WCu=0.6), and an upper limit of the parameter F2 obtained by the linear programming is 10.5 (WNi=1.0, WCu=0.6, and WCr=0.05).

Various components that can be further contained in the steel composition will be described.

“Ti having a content equal to or more than 0.0020 mass % and equal to or less than 0.0120 mass %.”

“N having a content equal to or more than 0.0050 mass % and equal to or less than 0.0250 mass %.”

The Ti and N generate fine Ti nitrides (or Ti carbonitrides). The fine Ti nitrides (or Ti carbonitrides) have effects of preventing austenitic grains from being coarsened when the crankshaft is hot-forged, accelerating precipitation of ferrite grains in the ferrite and pearlite structure after cooling, and making pearlite grains finer. The above effects enable to improve the fatigue strength without deteriorating the straightenability of the steel. If the Ti content is less than 0.0020 mass % or the N content is less than 0.0050 mass %, these effects cannot be exhibited. If the Ti content exceeds 0.0120 mass % or the N content exceeds 0.0250 mass %, coarse Ti nitrides are generated, to which concentrated stress is applied. As a result, the fatigue strength of the steel member is reduced. More preferably, the Ti content is equal to or more than 0.0030 mass % and equal to or less than 0.010 mass %. In addition, the N content is more preferably equal to or more than 0.007 mass % and equal to or less than 0.020 mass %. Whereas it is obviously possible to set the Ti or N content lower than the lower limit of the above composition ranges (such as omitting the intentional addition) depending on a required degree of the fatigue strength, thus enabling to receive the effects of improving the straightenability while maintaining the yield stress (or fatigue strength) at a certain degree.

“O having a content equal to or more than 0.0005 mass % and equal to or less than 0.008 mass %.”

The O and the Ti, Al, Si, Ca contained in the steel generate oxides, which serve as a precipitation nucleus of MnS, thereby dispersing MnS into the steel finely and uniformly. This MnS has effects of accelerating precipitation of intragranular ferrite to be precipitated in former austenite grains during cooling after hot-forging, uniformly reducing magnitudes of pearlite blocks, and improving the straightenability. If the O content is less than 0.0005 mass %, these effects cannot be exhibited. If the O content exceeds 0.008 mass %, an appropriate oxide composition cannot be obtained. The O content is more preferably equal to or more than 0.001 mass % and equal to or less than 0.005 mass %.

“If the contents of the O, N, and Ti are represented by WO, WN, and WTi mass %, respectively, the WO, WN, and WTi preferably satisfy: 0.12WTi<WO<2.5WTi  (3) and 0.04WN<WO<0.7WN  (4).”

According to the expressions (3) and (4), if the WO is equal to or lower than a lower limit, then the amount of oxides that serve as the precipitation nucleus of the MnS is reduced, and the MnS cannot be dispersed into the steel finely and uniformly. If the WO is equal to or higher than an upper limit, then Ti oxides are excessively generated, less nitrides are generated, and the growth of the former austenite grains cannot be sufficiently suppressed during the hot-forging. However if Ti or N is not intentionally added as mentioned before, the preferable scope of O or N contents described above is not necessary.

“Ca having a content equal to or more than 0.0005 mass % and equal to or less than 0.0050 mass %.”

The Ca has an effect of improving the machinability. To produce this effect conspicuously, it is essential for the Ca content to be equal to or more than 0.0005 mass %. If the Ca is excessively added so that the Ca content exceeds 0.0050 mass%, a large amount of high melting point CaS is generated, resulting in serious obstacle to a molten steel casting step. It is obviously possible to omit intentional addition of Ca, when the machinability improvement is not so required.

“Bi having a content equal to or more than 0.01 mass % and equal to or less than 0.10 mass %.”

“Te having a content equal to or more than 0.01 mass % and equal to or less than 0.10 mass %.”

“Pb having a content equal to or more than 0.01 mass % and equal to or less than 0.10 mass %.”

Similarly to the Ca, the Bi, the Te, and the Pb can be used as machinability improving elements. It is obviously possible to omit intentional addition of the above elements, when the machinability improvement is not so required.

Allowable ranges of impurity elements will be described. It is noted that the impurity elements are generic terms of sub components other than the above-stated elements, which components are allowed to be contained in the steel within ranges within which the impurity elements do not adversely influence the expression of the effects of the present invention. The impurity elements include the inevitable impurities inevitably mixed into the steel during manufacturing and elements added to the steel intentionally.

“Al having a content (a solid-solution concentration in the bulk steel) equal to or less than 0.045 mass % (including zero mass %).”

If the Al is contained in the steel, then nitrides are precipitated in the nitrocarburized layer, the surface hardness is considerably increased, and the straightenability is deteriorated. Therefore, the Al content is desirably as low as possible and preferably equal to or less than 0.025 mass %, more preferably equal to or less than 0.010 mass %.

“P having a content equal to or less than 0.010 mass % (including zero mass %).”

Since the P reduces an impact value, the P content is preferably as low as possible.

“Mo having a content equal to or less than 0.05 mass % (including zero mass %).”

Since Mo deteriorates the straightenability, the Mo content is preferably as low as possible.

“H having a content equal to or less than 0.01 mass %.”

Since the H causes a delayed fracture or the like, the H content is preferably as low as possible.

Allowable contents of the other elements are as follows. Since inclusion of rare-gas elements, artificial elements, and radioactive elements in the steel is not practical, they are excluded below.

“Li, Na, K, Rb, Cs, and Fr each having a content equal to or less than one ppm.”

“Be, Mg, Sr, and Ba each having a content equal to or less than one ppm.”

“Sc, Y, Ra, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Eb, Tm, Yb, and Lu each having a content equal to or less than 0.1 mass %.”

“Zr and Hf each having a content equal to or less than 0.1 mass %.”

“V, Nb, and Ta each having a content equal to or less than 0.1 mass %.”

“W having a content equal to or less than 0.1 mass %.”

“Tc and Re each having a content equal to or less than 0.01 mass %.”

“Ru and Os each having a content equal to or less than 0.01 mass %.”

“Co having a content equal to or less than 0.1 mass %.”

“Rh, Pd, Ag, Ir, Pt, and Au each having a content equal to or less than 0.01 mass %.”

“Zn, Cd, and Mg each having a content equal to or less than 0.01 mass %.”

“B having a content equal to or less than 0.005 mass %.”

“Ga, In, and Tl each having a content equal to or less than 0.01 mass %.”

“Ge and Sn each having a content equal to or less than 0.1 mass %.”

“As and Sb each having a content equal to or less than 0.1 mass %.”

“Se and Po each having a content equal to or less than 0.1 mass %.”

“F, Cl, Br, I, and At each having a content equal to or less than 0.1 mass %.”

“In the nitrocarburized microalloyed steel member according to the present invention, it is preferable that a Vickers hardness at a position 0.05 millimeters away from an uppermost surface of the nitrocarburized layer is equal to or higher than 280 Hv and equal to or lower than 380 Hv.”

If the Vickers hardness is lower than 280 Hv, the sufficient fatigue strength cannot be secured. If the Vickers hardness exceeds 380 Hv, the sufficient straightenability cannot be secured. The Vickers hardness is more preferably equal to or higher than 300 Hv and equal to or lower than 375 Hv. A formation thickness of the nitrocarburized layer is set preferably equal to or larger than 0.1 millimeters and equal to or smaller than 2.0 millimeters, more preferably equal to or larger than 0.5 millimeters and equal to or smaller than 1.5 millimeters.

“In the nitrocarburized microalloyed steel member according to the present invention, it is preferable that a ferrite area percentage in an area of the ferrite and pearlite structure is equal to or more than 20 percent and equal to or less than 60 percent.”

If the ferrite area percentage is set equal to or more than 20 percent, the straightenability can be improved. If the ferrite area percentage exceeds 60 percent, the fatigue strength is often insufficiently low. The ferrite area percentage is more preferably equal to or more than 30 percent and equal to or less than 55 percent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of one example of a crankshaft according to the present invention;

FIG. 2 is a typical representation of a stress-strain curve;

FIG. 3 is a projected plan of composition ranges of Cu, Cr, and Ni of a microalloyed steel according to the present invention;

FIG. 4 is a graph of plotting fatigue strengths obtained as a test result relative to values of F1; and

FIG. 5 is a graph of plotting crack initiation strokes obtained as a test result relative to values of F2.

DESCRIPTION OF THE PRESENT INVENTION

FIG. 1 is a front view of one example of a crankshaft according to the present invention. The crankshaft 1 is configured so that crank arms 2 arranged at predetermined intervals in a direction of a rotational axis O are alternately coupled by a crank journal 4 arranged so that a central axis of the journal 4 coincides with the rotational axis O, and by crank pins 5 each having a central axis at a position away from the rotational axis O by a certain distance in a radial direction A hole 8 for injecting oil is formed in each crank pin 5. The crank arms 2 form a proximal surface formation portion in which a surface of each crank arm 2 which surface faces the adjacent crank arm 2 is a flat proximal surface 2 a. A fillet 7, an outside diameter of which is gradually larger as closer to the proximal surface 2 a side, is formed on a protruding proximal end of the crank journal 4 and the crank pin 5 (an axis portion). The protruding proximal end is concave, so that a stress tends to concentrate thereon when a bending load acts thereon. However, by forming the fillet 7, the concentration of the stress is relaxed and a bending strength of the crankshaft 1 can be increased.

Each of the crank journal 4 and the crank pin 5 is formed into an axis having a circular cross section. After the steel having the composition stated above is hot-forged, a nitrocarburized layer is formed on an entire outer circumference of the steel. The crankshaft 1 thus configured is manufactured as follows. Materials are molten, cast, and divided into ingots so as to obtain the steel having the composition already described above in detail. Thereafter, each of the divided steel ingots is hot-forged and then air-cooled. By air-cooling the steel ingot in the atmospheric air, a ferrite and pearlite structure is obtained. Thereafter, the steel ingot is machined into a crankshaft by cutting without performing a normalizing treatment. After cutting, the resultant member is subjected to a nitrocarburizing treatment in an ammonia gas atmosphere. The member is then subjected to a well-known cold straightening treatment using a straightening roll or the like, thereby correcting deformations, strains or the like of the member generated during the nitrocarburizing treatment. As the steel adopted in the present invention, the steel having the above-stated composition is used. This can facilitate the straightening treatment after the nitrocarburizing treatment even if the normalizing treatment is omitted. In addition, despite such excellent straightenability, the steel exhibits excellent fatigue strength.

EXAMPLES

A result of an experiment conducted to confirm the effects of the present invention will be described.

Materials are mixed together to obtain steels having compositions shown in Table 1 (Examples) and Table 2 (Comparisons), respectively, and the steels are molten in an electric furnace into five-ton steel ingots. The O and N contents are adjusted by an amount of mixed gas of O₂ N₂ and Ar bubbled to the molten steels. Each of these steel ingots is hot-rolled into a rolled rod steel having a cross section of a 70-millimeter square. The rolled rod steel is heated to 1200 degrees Centigrade, held to be heated for 60 minutes and then hot-forged. Thereafter, the resultant rolled rod steel is subjected to natural cooling, thereby manufacturing a steel material having a 40-millimeter square cross section. A test piece is cut out from each steel material, and tested for a surface layer hardness, a straightenability, a ferrite area percentage, and a fatigue strength. The fatigue strength is measured as follows. The test piece having a length of 210 millimeters and a shape disclosed in FIG. 1 of the Patent Literature 6 is created from the materials obtained in the above. A gas nitrocarburizing treatment is performed on the test piece at 580 degrees Centigrade for 1.5 hours, and an Ono type rotating bending fatigue test is executed. This test is conducted on the test piece while varying a maximum load applied thereto, and the maximum load at which the test piece is not broken after being rotated ten million times is set as the fatigue strength of the test piece. The hardness is measured as follows. The same test piece as that used to measure the fatigue strength is created, and a sample for hardness measurement is cut off from an R part of the created test piece. In addition, the hardness of the sample at a position 0.05 millimeters away from a surface layer is measured by a Vickers hardness meter (with a load of 300 grams). The ferrite area percentage is calculated as follows. The ferrite and pearlite structure is mirror-polished and then etched by a picric acid, and a ferrite phase and a pearlite phase are discriminated on the etched structure by an image analysis. The straightenability is measured as follows. A three point bending test is conducted by applying a concentrated load to a central portion of each test piece while both ends thereof are supported at a supporting point distance of 182 millimeters. In this test, the concentrated load is continuously applied to the central portion until a crack occurs. A maximum deflection amount (crack initiation stroke) before occurrence of the crack is obtained as the straightenability of the test piece. The result is shown in the Tables 1 and 2 (In the comparisons, the blank cells in the columns of F1 or F2 in Table 2 indicate that their composition ranges exceed the range of the present invention, thus it is unable to apply F1 or F2 formula). TABLE 1 C Si Mn P S Cu Ni Cr F1 F2 Ti N Example 1 0.38 0.07 0.94 0.01 0.06 0.10 0.10 0.15 32.8 1.95 0.0030 0.0180 Example 2 0.38 0.10 0.65 0.08 0.10 0.10 0.10 0.15 32.8 1.95 0.0040 0.0200 Example 3 0.36 0.10 0.63 0.02 0.07 0.20 0.20 0.15 37.8 2.50 0.0030 0.0110 Example 4 0.35 0.15 0.64 0.02 0.08 0.49 0.49 0.15 52.3 4.10 0.0050 0.0120 Example 5 0.34 0.09 0.70 0.02 0.06 0.15 0.07 0.12 29.7 3.23 0.0050 0.0130 Example 6 0.39 0.18 0.70 0.02 0.09 0.25 0.15 0.12 34.7 3.70 0.0040 0.0080 Example 7 0.37 0.12 0.63 0.02 0.07 0.50 0.10 0.15 52.8 2.55 0.0040 0.0060 Example 8 0.36 0.10 0.63 0.02 0.08 0.10 0.50 0.15 32.8 3.55 0.0050 0.0070 Example 9 0.36 0.09 0.65 0.02 0.07 0.10 0.98 0.15 32.8 5.47 0.0050 0.0150 Example 10 0.34 0.18 0.73 0.01 0.09 0.24 0.15 0.15 39.8 2.36 0.0060 0.0140 Example 11 0.35 0.15 0.73 0.01 0.10 0.10 0.30 0.20 42.0 0.55 0.0030 0.0230 Example 12 0.36 0.19 0.74 0.01 0.10 0.29 0.12 0.20 51.5 0.12 0.0080 0.0200 Example 13 0.36 0.20 0.77 0.01 0.10 0.10 0.49 0.20 42.0 1.31 0.0070 0.0220 Example 14 0.36 0.20 0.75 0.01 0.11 0.29 0.29 0.19 49.7 1.24 0.0100 0.0140 Example 15 0.36 0.15 0.74 0.01 0.10 0.20 0.19 0.15 37.8 2.46 0.0110 0.0120 Example 16 0.34 0.25 0.74 0.01 0.10 0.05 0.05 0.15 30.3 1.68 0.0080 0.0130 Example 17 0.42 0.15 0.55 0.01 0.15 0.30 0.20 0.18 48.3 1.33 0.0070 0.0100 Example 18 0.33 0.10 0.60 0.01 0.04 0.30 0.20 0.06 26.1 6.61 0.0070 0.0150 Ferrite area Fatigue Surface layer Straightenability percentage strength O Ca hardness (Hv) (mm) (%) (MPa) Example 1 0.0020 0.0010 323 8.7 34.9 415 Example 2 0.0033 0.0020 320 7.1 41.8 419 Example 3 0.0025 0.0025 314 8.5 44.5 426 Example 4 0.0010 0.0033 326 8.9 42.7 433 Example 5 0.0035 0.0008 311 7.2 49.7 416 Example 6 0.0055 0.0011 302 9.9 40.1 415 Example 7 0.0040 0.0020 322 7.9 42.1 429 Example 8 0.0022 0.0035 314 8.9 42.6 425 Example 9 0.0026 0.0044 320 11.2 38.2 410 Example 10 0.0036 0.0009 329 7.8 46.9 426 Example 11 0.0046 0.0010 343 7.6 41.3 419 Example 12 0.0019 0.0015 343 6.8 40.0 441 Example 13 0.0029 0.0030 349 5.5 37.4 415 Example 14 0.0030 0.0026 363 5.2 39.1 436 Example 15 0.0050 0.0017 359 7.0 42.7 420 Example 16 0.0020 0.0026 313 8.5 49.0 398 Example 17 0.0025 0.0036 332 7.1 33.6 430 Example 18 0.0065 0.0010 280 11.7 55.6 402

TABLE 2 C Si Mn P S Cu Ni Cr F1 F2 Ti N Comparison 1 0.35 0.08 0.64 0.02 0.08 0.19 0.19 0.25 55.8 −1.96 0.0020 0.0030 Comparison 2 0.34 0.10 0.73 0.01 0.10 0.48 0.49 0.26 72.1 −0.76 0.0080 0.0140 Comparison 3 0.36 0.10 0.70 0.02 0.05 1.50 0.30 0.15 0.0020 0.0190 Comparison 4 0.20 0.10 0.70 0.02 0.05 1.20 1.00 0.10 0.0030 0.0040 Comparison 6 0.45 0.20 0.60 0.02 0.06 0.05 0.05 0.05 11.8 6.08 0.0100 0.0060 Comparison 6 0.35 0.20 0.60 0.02 0.06 0.10 0.10 0.25 51.3 −2.45 0.0010 0.0080 Comparison 7 0.42 0.20 0.70 0.02 0.05 0.20 1.50 0.20 0.0010 0.0080 Comparison 8 0.55 0.20 0.90 0.02 0.07 0.20 0.10 0.20 0.0030 0.0130 Comparison 9 0.40 0.20 1.50 0.01 0.05 0.20 0.20 0.20 0.0060 0.0120 Comparison 10 0.15 0.10 0.50 0.01 0.20 0.05 0.05 0.20 0.0080 0.0140 Comparison 11 0.40 0.20 0.30 0.01 0.05 0.20 0.20 0.10 0.0150 0.0150 Comparison 12 0.35 0.10 0.70 0.01 0.25 0.10 0.10 0.10 0.0010 0.0180 Comparison 13 0.40 0.20 0.55 0.02 0.10 0.20 0.30 0.40 0.0010 0.0120 Comparison 14 0.37 0.50 0.65 0.02 0.08 0.20 0.20 0.15 0.0010 0.0150 Comparison 15 0.36 0.79 0.64 0.02 0.08 0.20 0.20 0.15 0.0010 0.0080 Ferrite area Fatigue Surface layer Straightenability percentage strength O Ca hardness (Hv) (mm) (%) (MPa) Comparison 1 0.0020 0.0007 354 3.8 39.2 424 Comparison 2 0.0018 0.0010 385 3.2 35.3 463 Comparison 3 0.0020 0.0002 368 7.8 35.5 372 Comparison 4 0.0017 0.0003 336 9.8 64.3 381 Comparison 6 0.0015 0.0007 270 11.8 37.2 385 Comparison 6 0.0012 0.0004 361 3.1 42.7 419 Comparison 7 0.0009 0.0002 348 4.7 18.8 377 Comparison 8 0.0017 0.0001 362 3.5 1.6 385 Comparison 9 0.0011 0.0002 401 1.1 15.4 381 Comparison 10 0.0007 0.0006 327 6.7 86.2 390 Comparison 11 0.0010 0.0003 277 11.6 48.0 374 Comparison 12 0.0010 0.0002 298 10.0 50.5 370 Comparison 13 0.0011 0.0002 428 1.3 23.4 383 Comparison 14 0.0022 0.0002 354 4.9 45.0 384 Comparison 15 0.0023 0.0001 365 4.6 48.6 388

As evident from this result, either the straightenability or the fatigue strength of each of the crankshafts using the steels according to respective comparisons does not reach a necessary level. Each of the crankshafts using the steels according to respective examples of the present invention is, by contrast, excellent in both straightenability and fatigue strength. Ti, N and Ca are intentionally added to each example, however as stated above, it is obviously possible to omit containing the above elements (intentional addition: excluding inevitable impurities). When the Ti or N addition is omitted, comparing the results in Table 1, though the fatigue strength of each example is slightly deteriorated, it is practically acceptable, and the straightenability is achieved nearly equally. Ca only affects machinability, thus the omission of Ca intentional addition hardly affects the fatigue strength or the straightenability. FIG. 4 is a graph of plotting the fatigue strengths obtained as the test result relative to values of F1 of respective compositions. FIG. 5 is a graph of plotting the crack initiation strokes obtained as the test result relative to values of F2 of respective compositions. As can be seen from FIGS. 4 and 5, a correlation is recognized between the F1 or F2 and the test result. 

1. A nitrocarburized microalloyed steel member consisting of a microalloyed steel that includes a nitrocarburized layer on a surface, a cross-sectional structure of the steel except for the nitrocarburized layer including a ferrite and pearlite structure, wherein the microalloyed steel mainly consists of Fe and has a composition: C having a content equal to or more than 0.30 mass % and equal to or less than 0.50 mass %; Si having a content equal to or more than 0.05 mass % and equal to or less than 0.30 mass %; Mn having a content equal to or more than 0.50 mass % and equal to or less than 1.00 mass %; S having a content equal to or more than 0.03 mass % and equal to or less than 0.20 mass %; Cu having a content equal to or more than 0.05 mass % and equal to or less than 0.60 mass %; Ni having a content equal to or more than 0.02 mass % and equal to or less than 1.00 mass %; and Cr having a content equal to or more than 0.05 mass % and equal to or less than 0.30 mass %, and wherein if the contents of the Cu, the Ni, and the Cr are represented by WCu, WNi, and WCr mass %, respectively, and composition parameters F1 and F2 are 185WCr+50WCu and 8+4WNi+1.5WCu−44WCr, respectively, then the composition parameters F1 and F2 satisfy: F1>20  (1) and F2>0  (2).
 2. The nitrocarburized microalloyed steel member according to claim 1, wherein the microalloyed steel further contains: Ti having a content equal to or more than 0.0020 mass % and equal to or less than 0.0120 mass %; N having a content equal to or more than 0.0050 mass % and equal to or less than 0.0250 mass %; and O having a content equal to or more than 0.0005 mass % and equal to or less than 0.008 mass %.
 3. The nitrocarburized microalloyed steel member according to claim 2, wherein if the contents of the O, the N, and the Ti are represented by WO, WN, and WTi mass %, respectively, the WO, the WN, and the WTi satisfy: 0.12WTi<WO<2.5WTi  (3) and 0.04WN<WO<0.7WN  (4).
 4. The nitrocarburized microalloyed steel member according to claim 1, wherein the microalloyed steel further contains Ca having a content equal to or more than 0.0005 mass % and equal to or less than 0.0050 mass %.
 5. The nitrocarburized microalloyed steel member according to claim 1, wherein a Vickers hardness at a position 0.05 millimeters away from an uppermost surface of the nitrocarburized layer is equal to or higher than 280 Hv and equal to or lower than 380 Hv.
 6. The nitrocarburized microalloyed steel member according to claim 1, wherein a ferrite area percentage in an area of the ferrite and pearlite structure is equal to or more than 20 percent and equal to or less than 60 percent.
 7. The nitrocarburized microalloyed steel member according to claim 2, wherein the microalloyed steel further contains Ca having a content equal to or more than 0.0005 mass % and equal to or less than 0.0050 mass %.
 8. The nitrocarburized microalloyed steel member according to claim 3, wherein the microalloyed steel further contains Ca having a content equal to or more than 0.0005 mass % and equal to or less than 0.0050 mass %.
 9. The nitrocarburized microalloyed steel member according to claim 2, wherein a Vickers hardness at apposition 0.05 millimeters away from an uppermost surface of the nitrocarburized layer is equal to or higher than 280 Hv and equal to or lower than 380 Hv.
 10. The nitrocarburized microalloyed steel member according to claim 3, wherein a Vickers hardness at a position 0.05 millimeters away from an uppermost surface of the nitrocarburized layer is equal to or higher than 280 Hv and equal to or lower than 380 Hv.
 11. The nitrocarburized microalloyed steel member according to claim 4, wherein a Vickers hardness at a position 0.05 millimeters away from an uppermost surface of the nitrocarburized layer is equal to or higher than 280 Hv and equal to or lower than 380 Hv.
 12. The nitrocarburized microalloyed steel member according to claim 7, wherein a Vickers hardness at a position 0.05 millimeters away from an uppermost surface of the nitrocarburized layer is equal to or higher than 280 Hv and equal to or lower than 380 Hv.
 13. The nitrocarburized microalloyed steel member according to claim 8, wherein a Vickers hardness at a position 0.05 millimeters away from an uppermost surface of the nitrocarburized layer is equal to or higher than 280 Hv and equal to or lower than 380 Hv.
 14. The nitrocarburized microalloyed steel member according to claim 2, wherein a ferrite area percentage in an area of the ferrite and pearlite structure is equal to or more than 20 percent and equal to or less than 60 percent.
 15. The nitrocarburized microalloyed steel member according to claim 3, wherein a ferrite area percentage in an area of the ferrite and pearlite structure is equal to or more than 20 percent and equal to or less than 60 percent.
 16. The nitrocarburized microalloyed steel member according to claim 4, wherein a ferrite area percentage in an area of the ferrite and pearlite structure is equal to or more than 20 percent and equal to or less than 60 percent.
 17. The nitrocarburized microalloyed steel member according to claim 7, wherein a ferrite area percentage in an area of the ferrite and pearlite structure is equal to or more than 20 percent and equal to or less than 60 percent.
 18. The nitrocarburized microalloyed steel member according to claim 8, wherein a ferrite area percentage in an area of the ferrite and pearlite structure is equal to or more than 20 percent and equal to or less than 60 percent.
 19. The nitrocarburized microalloyed steel member according to claim 5, wherein a ferrite area percentage in an area of the ferrite and pearlite structure is equal to or more than 20 percent and equal to or less than 60 percent.
 20. The nitrocarburized microalloyed steel member according to claim 9, wherein a ferrite area percentage in an area of the ferrite and pearlite structure is equal to or more than 20 percent and equal to or less than 60 percent.
 21. The nitrocarburized microalloyed steel member according to claim 10, wherein a ferrite area percentage in an area of the ferrite and pearlite structure is equal to or more than 20 percent and equal to or less than 60 percent.
 22. The nitrocarburized microalloyed steel member according to claim 11, wherein a ferrite area percentage in an area of the ferrite and pearlite structure is equal to or more than 20 percent and equal to or less than 60 percent.
 23. The nitrocarburized microalloyed steel member according to claim 12, wherein a ferrite area percentage in an area of the ferrite and pearlite structure is equal to or more than 20 percent and equal to or less than 60 percent.
 24. The nitrocarburized microalloyed steel member according to claim 13, wherein a ferrite area percentage in an area of the ferrite and pearlite structure is equal to or more than 20 percent and equal to or less than 60 percent. 